1. Field of the Invention
The invention relates to a control system for a continuously variable transmission installed to a vehicle, and more particularly, to a control system for a continuously variable transmission of a type having a toroidal continuously variable mechanism.
2. Description of the Related Art
There have been known various continuously variable transmissions such as incorporating a toroidal continuously variable mechanism installed to an automobile vehicle. Such a toroidal type continuously variable transmission, which comprises an input and output toroidal disk and a roller disposed between the input and output toroidal disks, continuously varies a gear ratio according to inclinations of the roller relative to the toroidal disks. The roller is supported for rotation by a support member which is called a trunnion. In order to incline the roller the trunnion is shifted in a direction tangent to the toroidal surface of the toroidal disk by a hydraulically operated actuator. A shift of the trunnion, and hence an inclination of the roller, is controlled by hydraulic working pressure supplied to the hydraulically operated actuator through a shift valve of a double-slider type which has a hollow valve sleeve axially slidable in a valve body and a valve spool axially slidable in the valve sleeve.
The valve sleeve is shifted and changed in position relative to the valve spool by a stepping motor to regulate hydraulic working pressure supplied to the actuator so as to shift the roller through the trunnion in a direction tangent to the toroidal surface from a neutral position. As a result of the shift, the roller inclines relative to the toroidal disk to change a gear ratio between an input speed to the toroidal continuously variable mechanism and output speed from the toroidal continuously variable mechanism. The inclination of the roller is mechanically fed back to the shift valve to shift the valve spool in the same direction in which the valve sleeve has been shifted, as a result of which the shift valve returns to a balanced or neutral position. During the return of the shift valve to the neutral position, while the roller is left inclined, it is shift back to the neutral position and held in the neutral position.
As described in, for example, Japanese Unexamined Patent Publications Nos. 3-223555 and 6-101754, it has been known that what is called a geared neutral starting system is employed in the continuously variable transmission. In the toroidal type continuously variable transmission equipped with the geared neutral starting system, the toroidal continuously variable mechanism is mounted on a transmission input shaft connected to the engine, and a planetary gear mechanism is mounted on a secondary shaft in parallel to the input shaft. The planetary gear mechanism is comprised of three rotary elements, namely a sun gear, an internal gear and a pinion carrier supporting a pinion carrier meshed with the sun gear and the internal gear. One of these rotary elements, i.e. the internal gear is used as an transmission output gear. Rotation of an engine is imparted to the planetary gear mechanism partly directly through the pinion carrier and partly through the sun gear via the toroidal continuously variable mechanism.
The ratio of rotation between the pinion carrier and the sun gear is varied by controlling the gear ratio of the toroidal continuously variable mechanism so as to hold the transmission output element, i e. the sin gear, remain stand still, providing a neutral condition. By increasingly or decreasingly varying the gear ratio of the toroidal continuously variable mechanism causes the internal gear as the transmission output element to rotate in a forward direction or in a reverse direction.
Further, some toroidal type continuously variable transmissions equipped with the geared neutral starting system are switchable between what is called a high gear ratio control mode in which output rotation of the toroidal continuously variable mechanism is transmitted directly to the secondary shaft without the aid of the planetary gear mechanism and what is called a low gear ratio control mode in which rotation of an engine is transmitted directly to the pinion carrier and transmitted to the sun gear through the toroidal continuously variable mechanism. In such a toroidal type continuously variable transmission, in order to cause a switch between the high and low modes, the toroidal type continuously variable transmission is provided with a low mode clutch to connect and disconnect the input shaft and the pinion carrier of the planetary gear mechanism on the secondary shaft and a high mode clutch to connect and disconnect the toroidal continuously variable mechanism on the input shaft and the secondary shaft. These mode clutches are alternately locked and unlocked in a state where a gear ratio of the continuously variable transmission remains identical in both high and low modes.
As was previously described, after a shift in a direction tangent to the toroidal surface of the toroidal disks and inclination, the roller of the toroidal continuously variable mechanism is returned to its neutral position, i.e. a position of a plane perpendicular to the tangential direction when the plane passes a center axis of rotation of the toroidal disks, and is balanced in that position. However, the roller is practically balanced in a position slightly shifted on a specific side of the neutral position in the tangential direction.
Specifically, as shown in FIG. 24, when torque is transmitted, for example, from an input toroidal disk A to an output toroidal disk B through a roller D supported by a trunnion C (this direction of torque transfer is referred to as a normal direction) while the input and output toroidal disks A and B and the roller D are rotating in directions indicated by arrows A', B' and C', respectively, drag force from the input or drive toroidal disk A or reaction force of the output or driven toroidal disk B acts on the trunnion C as traction force T directed downward as viewed in the figure. Hydraulic working pressure is supplied to a roller actuator E to hold the trunnion C or the roller D in the neutral position against the traction force T. In this instance, the roller D is practically balanced in a position shifted according to input torque in a direction opposite to the traction force T (upward as viewed in the figure), i.e. in a direction in which the toroidal continuously variable mechanism changes its gear ratio toward the higher speed side (which is referred to as a plus side), from the neutral position. The shift direction depends upon the transfer direction of torque through the toroidal continuously variable mechanism. For example, when torque is transmitted in a direction to the input toroidal disk A from the output toroidal disk B (which is referred to as a reverse direction) while the input and output toroidal disks A and B and the roller D are rotating in directions indicated by arrows A', B' and C', respectively, the roller D is balanced in a position shifted downward as viewed in the figure, i.e. in a position shifted in a direction in which the toroidal continuously variable mechanism changes its gear ratio toward the lower speed side (which is referred to as a minus direction), from the neutral position. Accordingly, the roller D shifts instantaneously from one side to another side of the neutral position. The directional reversal of torque transfer occurs in response to a switch from a normal drive state in which the vehicle is driven by the engine to a reverse drive state in which the engine is driven by the vehicle running with inertia or vice versa.
In the toroidal type continuously variable transmissions switchable between the high and low gear ratio control modes, torque is transmitted from the output toroidal disk to the input toroidal disk in the low mode while the vehicle is in the normal drive state and from the input toroidal disk to the output toroidal disk in the high mode while the vehicle is in the normal drive state, and, on the other hand, from the input toroidal disk to the output toroidal disk in the low mode while the vehicle is in the reverse drive state and from the output toroidal disk to the input toroidal disk in the high mode while the vehicle is in the reverse drive state. The directional reversal of torque transfer occurs also in response to a switch between the low and high gear ratio control modes. An instantaneous shift, and hence an instantaneous inclination, of the roller is accompanied by an occurrence of shocks.